The steps below can be used to add many people to a group at once.
On your iPhone, open the Contacts app.
In the upper-left corner, select "Groups" by clicking.
To add contacts, tap the group you want.
To add contacts to the group, tap the plus sign (+) in the group's upper-right corner.
Then touch "Done" after selecting the contacts you want to put in the group.
Read More:- https://ityug247.com/how-to-create-a-group-on-iphone/